The present invention relates to an image sensor comprised of a number of solid-state light sensor elements arranged to form a matrix of the light sensor elements each representing a unit pixel, wherein sensor signals from respective pixels are read out (scanned) in a time series by sequentially selecting the pixel lines one at a time and sequentially selecting each of the pixels in a selected line.
Japanese Laid-Open Patent Publication No. 2000-329616 discloses a conventional image sensor, in which a light sensor circuit representing a unit pixel comprises, as shown in FIG. 1, a photo-diode PD operating as a photoelectric converting element for producing therein a sensor current proportional to the quantity of incident light Ls falling thereon, a transistor Q1 having a logarithmic output characteristic in a weak inverse state for converting the sensor current produced in the photodiode PD into a voltage signal Vpd by using the property of its sub-threshold region, a transistor Q2 for amplifying the voltage signal Vpd and a transistor Q3 for outputting a sensor signal So in accordance with a timing pulse of a readout signal Vs and which is characterized by its wide dynamic range obtained by giving the output a logarithmic characteristic, thereby achieving the high sensitivity of detecting a light signal. This image sensor can change drain voltages of the transistors for all pixels from a normal to a lower level for a specified time to remove charges accumulated in parasitic capacitors C of respective photoelectric converting elements, thereby initializing all pixels before detecting light signals. Therefore, even if the sensor current rapidly decreased with a decreased illumination, each sensor circuit may immediately obtain a voltage signal corresponding to the incident light quantity at that moment, thereby eliminating the possibility of occurrence of afterglow of the pixel at a decreased quantity of incident light.
FIG. 2 shows a time chart of signals produced at respective portions of the light sensor circuit of FIG. 1. In FIG. 2, t1 is an initializing timing pulse, t2 is a timing pulse for outputting a light sensor signal So and T designates a period for accumulating a charge in a parasitic capacitor C of the photodiode PD.
FIG. 3 shows an output characteristic of a sensor signal So with a sensor current proportional to a quantity of light falling on the light sensor circuit of FIG. 1, which is characterized by a logarithmic output characteristic at a sufficient quantity of the sensor current flowing in the photodiode PD and by a non-logarithmic linear characteristic at a small quantity of the sensor current due to a response delay in charging a parasitic capacitor C. In FIG. 3, WA indicates a non-logarithmic response characteristic region and WB indicates a logarithmic response characteristic region.
FIG. 4 shows an exemplary construction of a conventional image sensor comprising a number of the above-described light sensor circuits arranged to form a matrix of pixels (i.e., light sensor circuits), wherein sensor signals from respective pixels are read by scanning in a time series.
The image sensor is composed of 4×4 pixels D11-D44 arranged in a matrix of pixel circuits, in which, under control of an electronic control unit ECU (not shown), pixel lines are selected one by one with respective selecting signals LS1-LS4 sequentially output from a pixel line selecting circuit 1 and pixels in each selected pixel-line are readout one by one as respective sensor signals So in such a manner that selecting signals DS sequentially output from a pixel selecting circuit 2 turn ON corresponding switches SW1-SW4 to read sensor signals So in a time series. Each of the sensor signals So sequentially readout from respective pixels is converted into a specified voltage signal Vo by applying a bias voltage Vcc through a reference resistance R. In FIG. 4, numeral 4 designates a power source for gate voltage VG of the transistor Q1, numeral 6 designates a power source for a drain voltage VD of the transistor Q1 and numeral 5 designates a voltage switching-over circuit 5 by which a drain voltage VD of each transistor Q1 for each pixel is changed to a normal high-level H or an initializing lower level L by the effect of a specified timing pulse.
FIG. 5 is a time chart of signals generated at respective portions of the thus constructed image sensor. However, the above-described conventional image sensor has a low signal driving ability to convert a sensor signal So from each pixel into an output voltage signal Vo and output it. In other words, if the image sensor reads out a sensor signal at an increased scanning speed, the voltage signal Vo would be output as unsaturated. For this reason, it cannot achieve high speed reading-out (scanning) of pixel signals.
To improve the signal driving ability of the conventional image sensor, it is modified by providing each pixel output of one pixel line with a bias circuit 11 which, as shown in FIG. 6, converts each sensor signal So read from each of respective pixels in a pixel line into a voltage signal Vo by applying a bias voltage Vcc through a reference resistance. This modification makes it possible to read out pixel signals at an increased scanning speed but, at the same time, increases power consumption for supplying electric current to a number of reference resistances R provided for each of the pixel outputs of each pixel line. High speed scanning is still difficult since the capacities of a plurality of analog switches (transistor switches) must be driven until a voltage signal Vo of the pixel selected by the pixel selecting circuit 2 is output.
Accordingly, an attempt has been made to improve signal driving ability of the image sensor by further providing a buffer circuit 12 which comprises, as shown in FIG. 15, buffer amplifiers BF connected to each one of the signal lines on the output side of the bias circuit 11 and which can temporally and intensively store voltage signals Vo from respective pixels selected by the pixel selecting circuit 2. The image sensor thus constructed, however, has such a disadvantage that the power consumption is further increased for supplying electric current to a number of buffer amplifiers with one provided for each of the pixel outputs of one pixel line.
Thus, the conventional image sensor comprising a number of solid-state light sensitive elements arranged to form a matrix of pixels, which is capable of reading sensor signals from respective pixels in a time series by sequentially selecting pixel lines one at a time and sequentially selecting pixels one by one in a selected pixel line and then outputting each sensor signal as a voltage signal obtained by applying a bias voltage through the reference resistance and which, however, cannot achieve high speed reading of pixel signals because of its low signal driving ability.
The image sensor, which is further provided with bias circuits disposed for each of outputs of each pixel line for converting a voltage signal from each pixel by applying a bias voltage through a reference resistance so as to improve the signal driving ability necessary for achieving high-speed reading of pixel signals, has to supply electric energy to a number of reference resistances R provided one for each of the pixel outputs of one pixel line, resulting in a considerable increase in power consumption.
The image sensor comprising a matrix of solid-state light sensor elements each representing a unit pixel, which sequentially reads signals from respective pixels in a time series by sequentially selecting pixel lines one at a time and by sequentially selecting pixels in a selected pixel line and which is provided with a bias circuit for each of the pixel outputs of one pixel line to convert respective pixel signals to respective voltage signals by applying a bias voltage through a reference resistance and which is further provided with a buffer circuit comprising buffer amplifiers connected to each one of the signal output lines to temporally accumulate voltage signals of respective pixels from the bias circuits and selectively output the voltage signals so as to improve the signal driving ability and achieve high-speed scanning of respective sensor signals, has a disadvantage of further increasing power consumption by supplying electric current to a number of reference resistances and a number of buffer amplifiers disposed one for each of the pixel outputs of one pixel line.